There has been generally used a heat roll fixing method for the electrophotography method in copiers and printers. Concretely, there is used a method which comprises forming an electrostatic latent image on the photo-sensitive material, then developing the latent image using a toner, transferring the toner image onto a fixing sheet such as paper or the like, and heat-pressing the transferred toner image using a heat roll. In this heat roll fixing method, in order to achieve enhanced economical efficiency in power consumption and increased photography speed, and to prevent papers from being curled, a toner superior in fixing properties is needed, which can be fixed at a lower temperature. In the conventional technique, there has been proposed a method to improve the molecular weight and molecular weight distribution of a binder resin. Concretely, it has been tested to have a binder resin of a low molecular weight and to lower the fixing temperature.
However, the melting point has been lowered owing to the lowered molecular weight whereas the cohesive force of a resin was also deteriorated simultaneously. Therefore, there was a problem such that an offset phenomenon occurred to the fixing roller. In order to prevent such a problem, there has been attempted a method to use a binder resin having a wide molecular weight distribution by mixing a high molecular weight resin and a low molecular weight resin, or, further to crosslink the high molecular weight portion of the binder resin or the like. However, in response to a demand for low temperature fixing properties in recent copiers, it is hard to mention that this is a technique capable of providing a toner having sufficient performance yet.
On the other hand, as copiers and printers have come to employ a higher speed, high performance requirements in electrified portions have also increased. That is, a higher durability is required for a toner and a longer-term stability in printing is needed.
Various resins for toners obtained by blending a linear polymer with a copolymer have been proposed as a binder resin requiring such performance conflicting with each other (for example, Patent Documents 1 and 2).
In Patent Document 1, there has been proposed a method for producing a resin for toners having a gel content by reacting a diisocyanate compound with a mixture of a copolymer comprising an acrylic or methacrylic ester monomer and a monomer having active hydrogen as a constituent unit, and a linear polymer, in order to prevent the offset phenomenon and exhibit a wide fixing temperature range. However, there were still problems in this method such that the gel fraction of the obtained resin was high and the fixing temperature became high.
Furthermore, in Patent Document 2, both low temperature fixing properties and hot offset resistance are achieved by having a molecular weight distribution of the binder resin and a molten viscosity at a high temperature and a low temperature of the binder resin within in a specific range.
However, various methods have been attempted heretofore in order to provide a resin composition for toners having sufficient fixing properties and offset resistance, and excellent in the productivity; however, no perfect methods could be obtained heretofore to fully overcome the above defects.
Patent Document 1: Japanese Patent Laid-open No. 1988-220170
Patent Document 2: Japanese Patent Laid-open No. 2001-92182